Hydrogen atom Adducts

For aqueous solutions, Sargent and Gardy (1976) have advocated the use of DMPO, which gives persistent spin adducts with hydroxyl radicals hydrogen-atom adducts arise by electron scavenging and subsequent protonation of DMPCK, but this can be minimized by incorporating N20 in the system, which traps electrons and raises the yield of hydroxyl radicals (28). 

In a more recent example, Uda et al. (1979) have used PBN to detect adsorbed hydrogen atoms formed on zinc oxide which has been exposed to an atmosphere of hydrogen gas. In order to obtain the PBN-hydrogen atom adduct, it was apparently necessary to suspend the exposed zinc oxide in a benzene solution of PBN, remove the solvent, and then extract with a fresh portion of benzene. It must be pointed out that, whilst infrared evidence indicated the formation on the solid of a species with a Zn—H bond, which was presumed to react with the trap with hydrogen atom character , reaction as H followed by oxidation would lead to the same result. 

Syrstad EA, Vivekananda S, Turecek F (2001) Direct observation of a hydrogen atom adduct to C-5 in uracil. A neutralzation-reionization mass spectrometric and ab initio study. J Phys Chem A... 

The +NR+ spectrum of 8 showed a small survivor ion, but differed substantially from the spectra of other C2H5NO isomers, e.g., 6, 7, AT-methylamino(hy-droxy)carbene (9), and N-methylformamide (10). The low intensity of survivor ions in the NR mass spectra of enol imines is due to Franck-Condon effects in collisional reionization that result in vibrational excitation of the resulting cation radical followed by dissociation. Franck-Condon effects were studied for collisional ionization of acetimidic acid, CH3C(OH)=NH, which was one of the neutral dissociation products of 1 -hydroxy- 1-methylamino-l-ethyl radical, a hydrogen atom adduct to AT-methylacetamide [37]. The cation-radical dissociates extensively upon reionization, and the dissociation is driven by a 74 kj mol-1 Franck-Condon energy acquired by vertical ionization. 

When hydrated electrons react with uncharged vinyl monomers, they may in principle give rise directly to hydrogen atom adducts ... 

Figure 8. Formation of hydrogen atom adduct in N2-satu-ratea solution at pH 2...

Direct strand breaks or alkali-labile lesions are not observed from 76,80 or 83 under anaerobic conditions. (Reminder Radicals are referred to using the same descriptor whether they are nucleosides or in biopolymers.) Small amounts of direct strand breaks are detected at the 5 -adjacent nucleotide when the formal C6-hydrogen atom adduct of thymidine (80) is produced under aerobic conditions. " Strand scission is reduced approximately fourfold when the 5 -adjacent nucleotide is deuterated at the Cl -position. " Deuteration of the d -, or C4 -positions has no effect suggesting that per-oxyl radical 86 selectively abstracted the Cl -hydrogen atom from the 5 -adjacent nucleotide (Scheme 36). Additional support for intemucleotidyl Cl -hydrogen atom abstraction was obtained when the tandem lesion containing 2-deoxyribonolactone (15) was observed in low yield when 80... 

Independent generation of C6-radicals 76 and 83 (Scheme 35) failed to yield any direct strand scission under aerobic conditions.These radicals do produce a variety of alkah-labile tandem lesions that were detected by denaturing polyacrylamide gel electrophoresis and mass spectrometry. The peroxyl radical of the formal C5-hydrogen atom adduct of 2 -deoxyuridine (87) produces tandem lesions by reacting with the... 

The formal C5- (91) and C6-hydrogen atom adducts (92) of uridine have been generated from the respective i-butyl ketones (93, 94, Scheme 39) 61-264 strand breaks were observed at the 5 -adjacent... 

The reduction in the butadiene formation with increasing H2S concentration reflects the change in the radical decomposition reaction (Figure 8). Hydrogen atom adducts abstract hydrogen from the H2S or other hydrocarbon molecules at rates which are competitive with, or faster than, those for radical decomposition 6) Therefore, products such as linear pentenes are formed in preference to butadiene. 

A radically different course is followed when the reaction of 2-alkyl-substituted thiazoles is periormed in methanol or acetonitrile (335), 2 1 adducts containing seven-membered azepine rings (91) are being formed in which two of the original activated hydrogen atoms have altered positions (Scheme 55). A similar azepine adduct (92) was obtained by... 

Adducts of BF and some organic compounds having labile hydrogen atoms in the vicinity of the atom bonding to the boron atom of BF may form a derivative of BF by splitting out HF. For example, P-diketones such as acetylacetone or benzoylacetone react with BF in benzene (38) ... 

This last result bears also on the mode of conversion of the adduct to the final substitution product. As written in Eq. (10), a hydrogen atom is eliminated from the adduct, but it is more likely that it is abstracted from the adduct by a second radical. In dilute solutions of the radical-producing species, this second radical may be the adduct itself, as in Eq. (12) but when more concentrated solutions of dibenzoyl peroxide are employed, the hydrogen atom is removed by a benzoyloxy radical, for in the arylation of deuterated aromatic compounds the deuterium lost from the aromatic nucleus appears as deuterated benzoic acid, Eq. (13).The over-all reaction for the phenylation of benzene by dibenzoyl peroxide may therefore be written as in Eq, (14). 

It is also interesting that the yield of monomethyl succinate (R — H), which presumably arises from the abstraction of a hydrogen atom by the aeyloxy radical, is higher in the presence of aromatic than aliphatic substrates. This may mean that the aeyloxy radical is mainly responsible for the abstraction of a hydrogen atom from the initial adduct (26) formed between the alkyl radical and the aromatic substrate (cf. Section II,B). 

Fig. 2. Geometries calculated (solid lines) and observed (bold dashed lines) for 1-propanol in its a-cyclodextrin adduct. G3 and G6 denote the numbers of glucopyranose units of a-cyclodextrin. H3 and H5 refer to the hydrogen atoms located inside of the cyclodextrin cavity. The hydrogen atoms for the observed geometry of 1-propanol are not shown, since their atomic coordinates have not been determined. The observed 1-propanol is twofold disordered, with site a occupied 80%, site b 20%. Interatomic distances are shown in bold italics on fine dashed lines (nm). Reproduced with permission from the Chemical Society of Japan...

The Mayo mechanism involves a thermal Diels-AIder reaction between two molecules of S to generate the adduct 95 which donates a hydrogen atom to another molecule of S to give the initiating radicals 96 and 97. The driving force for the molecule assisted homolysis is provided by formation of an aromatic ring. The Diels-AIder intermediate 95 has never been isolated. However, related compounds have been synthesized and shown to initiate S polymerization."110... 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

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